Printed circuits are used as a replacement for wiring when connecting electric components in an intricate electrical system. Printed circuits save space, weight and labor, and increase reliability of the circuit compared with round wire connections. Instead of metal round wire, the conventional printed circuit comprises a metal foil, generally copper, which is adhered to a base, typically fiberglass or epoxy, to form a metal-clad layer. The adhered metal foil is then imaged and etched into a desired pattern to form a circuit.
A particular type of printed circuitry, known as flexible printed circuits or "Flex", has the same basic structure as conventional hard printed circuit boards. Flex, however, uses a base film made of a flexible, dielectric material such as plastic. The dielectric substrate is typically a polyimide or polyester.
The flexible circuit contains at least one layer of metal foil bonded to a substrate with an adhesive. However, the complexity of most circuit designs requires that multiple layers be used. In the case of multilayer circuits, adhesives are required between multiple layers. In addition, flexible circuits generally require coverlays which are films comprised of an insulating material coated with adhesive. Commonly used materials for the insulating film include polyester and polyimide. Conventional adhesives for bonding the coverlays include polyester, acrylic and epoxy.
Flexible circuits are widely used in both the consumer and military markets, in specific applications where it has design advantages over standard hardboard and round wire technology. The plastic film, generally 25-75 microns thick, allows electronic packaging to be very thin and physically conform to the application. In aerospace applications, flexible circuits are used primarily to save space and weight. In consumer applications such as cameras, mobile phones and PCMCIA cards for notebook computers, flexible circuits serve the same purpose--saving space and weight. In computer disk drives, flexible circuits are used for their flexibility, allowing the cycling of the read/write head across the hard drive many millions of times.
The metal foil can be adhered to the base film by using adhesives or by using non-adhesive techniques. Adhesiveless laminates, in which the metal is directly bonded to the substrate without the use of an adhesive, have been proposed to decrease circuit thickness, increase flexibility and enhance thermal conductivity. Such techniques include plating and vapor or sputter deposition. It has been found, however, that such techniques suffer from a slow rate of manufacturing and high cost.
Conventional adhesives used to bond metal foil to dielectric substrates include polyimides, polyesters, modified epoxies, acrylics, and fluorocarbons. Polyimide adhesives provide a low coefficient of thermal expansion between the dielectric substrate and adhesive layers, as well as good electrical properties, and good chemical and thermal resistance. However, they require very high processing temperatures and are expensive. Polyester adhesives are flexible, have good chemical resistance and electrical properties, but only fair thermal resistance. Modified epoxies have thermal resistance and good electrical properties, but only fair chemical resistance and flexibility. Acrylic adhesives offer thermal and chemical resistance, electrical properties, and good flexibility. Fluorocarbons offer good thermal resistance and electrical properties, as well as excellent chemical resistance and flexibility, but poor dimensional stability.
The adhesives that bond the metal conducting foil to the substrate film, as well as adhesives that bond multiple layers together and cover the finished circuit from the environment, have proven to be the weak link in flexible circuit manufacturing and use. None of the conventional adhesives possess the combination of high temperature stability, high strength, and resistivity demanded by the industry at a reasonable cost.